The technology disclosed in Patent Document 1 is an example of a conventional electric power steering device. With this technology, when an abnormality occurs in one phase of a three-phase brushless motor, an abnormality electric current instruction value for keeping motor torques almost constant is calculated based on back electromotive voltage information of the remaining two phases so that the motor is continuously driven based on the above abnormality electric current instruction value.
However, the amount of electric current passed through the motor is limited by restrictions such as hardware and the like. Therefore, a constant motor torque cannot be obtained in the angular area in which the electric current limit is reached, so the driver feels as if the steering wheel were caught by something, thereby deteriorating the steering feeling.
There is a technology of providing an over-assist characteristic in a smaller angular area than the angular area in which a constant torque is not obtained so as to increase the motor torque, and providing an under-assist characteristic in a larger angler area than the angular area in which the constant torque is not obtained so as to decrease the motor torque (See Patent Document 2, for example). This technology aims at accelerating the rotation of the motor before reaching the angular area in which the constant torque is not obtained so as to pass over this angular area.